Transcript Temperature of Stars PP

 Missing
Lens
 Homework due Friday
 Physics Club
 SLC on Thursday
 Test next week
 Be
able to calculate the temperature of very
distant stars.
 Be
able to use the Doppler Effect to explain
how distant galaxies are moving relative to
Earth.
 Understand
how we know if the universe is
expanding or not.
How do we figure out the temperature
of a star?
 Do
you think all stars have the same
temperature?
 How
would you try to figure out the
temperature of a star?
 “Thermal
radiation, every body does it!”
 Thermal
radiation is any type of
electromagnetic radiation from an object
that depends on its temperature. Sometimes
called Blackbody radiation
 These
photons get their energy from heat not
from electrons dropping energy levels so
thermal radiation spectrums are the most
common type of continuous spectra because
of the large variation of photon energies.
Note: the wavelength stay in the ultraviolet to infrared range.
 Law
1 (Stefan-Boltzmann Law): Each square
meter of a hotter object’s surface emits
more light at all wavelengths.
 Law
2 (Wein’s Law): Hotter objects emit
photons with a higher average energy.
 What
happens when you heat up metal in a
furnace.
 Hot plate demo
 Let’s

ask my close friend Randy McDaniel!
http://www.youtube.com/watch?v=e_uO7PU8l8Y
 When
the sword is at a lower temperature it
only emits infrared light. When it is heated
it starts to emit visible light and ultraviolet
light.
 He
came up with an equation that related
the peak wavelength with temperature.
 Using
this equation all we need to do is find
the peak wavelength of a star and we can
calculate its temperature.
 If
people are constantly emitting thermal
radiation why don’t we see it?
 How
do night vision goggles work?
 How
can we tell the temperature of a planet
without visiting it?

Planets do not generate light like a stars do.
They are heated by the light they absorb, not by
the light they reflect.
 If
the peak frequency of a star is
5.20x10^14Hz find its temperature using
Wein’s law:
Green-Yellow!!
 If
the peak frequency of a star is
1.2x10^15Hz find its temperature using
Wein’s law:
 If
this peak frequency is in the ultraviolet
part of the electromagnetic spectrum can we
see the star with our own eyes?
 If
the peak frequency of a star is
1.4x10^14Hz find its temperature using
Wein’s law:
T=1353K
 If
this peak frequency is in the infrared part
of the electromagnetic spectrum can we see
the star with our own eyes?
 Is
there anything else you think we can learn
about the universe from electromagnetic
waves?